1. Field of the Invention
The present invention relates generally to an optical fiber, and more particularly to an apparatus and method for fabricating an optical fiber having a plurality of air holes.
2. Description of the Related Art
A holey optical fiber consists of a plurality of submicron-sized air holes running the length of a silica fiber in the cladding layer to confine light (by modified total internal reflection) to a core layer. The holey optical fiber finds its increased applications as a waveguide with novel properties for dispersion compensated fiber communications, non-linear fiber and grating applications, and optical fiber amplifications.
Basically, the holey optical fiber includes a dielectric structure with a refractive index that varies periodically across a transverse plane but is uniform in the normal direction. This dielectric structure causes Bragg diffraction and allows the holey optical fiber to have a “photonic band gap” at a specific wavelength or with respect to a light wave propagation direction. The “photonic band gap” refers to a condition in which light of certain frequencies will not propagate in the material and is analogous to the familiar electronic band gap, except that it applies to photons instead of electrons.
Accordingly, light can behave in unfamiliar ways, traveling along the holey optical fiber due to the photonic band gap effect and the reflective index characteristics. For details, see T. A. Birks, et. al., Electronic Letters, vol. 31(22), p. 1941, October, 1995 and J. C. Knight, et. al., Proceeding of OFC, PD 3-1, February, 1996.
In the conventional method of fabricating the holey optical fiber, an optical fiber preform is typically formed by arranging a plurality of hollow cylindrical glass tubes with a predetermined form along the cladding layer. At the same time, a core preform rod is inserted to be used depending on the application purposes, i.e., as an optical fiber amplifier, an optical fiber grating, or a non-linear optical fiber. Thereafter, the ends of the glass tubes are sealed, then the optical fiber is drawn from the preform. Accordingly, the resulting optical fiber has a plurality of submicron-sized air holes in the cladding.
In the conventional holey optical fiber, however, the outer air holes are typically closed or are much smaller than the inner air holes of the fiber. Hence, during the drawing of an optical fiber from the preform, relatively large inner air holes are transformed to an oval shape since the outer glass tubes are melted faster than the inner glass tubes due to the difference in the heat conductivity between the inner portion and the outer portion of the optical fiber preform. This type of distortion in the air holes makes the continuous mass production of holey optical fibers very difficult.